Additional Widget Option '-steadymarks'

This option needs a motivation:

The text widget has an odd behavior, marks will be shuffled when re-setting the gravity, and also when deleting a range of characters.

At first a simple example to demonstrate the shuffle effect. Try this script:

This is the output:

I've tried to build a minimal example (derived from a practice case) to demonstrate, why this shuffle effect is causing headaches:

When executing this script the text widget will show the content "2". This result is not intuitive (in sense of programmed control), the content "12" is expected. Is something wrong with the widget? We will do a proof. The basis for the proof is the definition from the Tk Documentation:

Each mark also has a "gravity", which is either left or right. The gravity for a mark specifies what happens to the mark when text is inserted at the point of the mark. If a mark has left gravity, then the mark is treated as if it were attached to the character on its left, so the mark will remain to the left of any text inserted at the mark position. If the mark has right gravity, new text inserted at the mark position will appear to the left of the mark (so that the mark remains rightmost). The gravity for a mark defaults to right.

Now we will examine the algorithm step by step, applying the rules of the Tk documentation:

(Note that the insert cursor and the ending newline are not of interest here.)
(We append the gravity to each mark for a better understanding.)
("cur" is denoting the current mark.)

In fact the content "2" is ok, everything is right. The rules of gravity are forcing a shuffle of the marks, they will in general not keep the defined order, this is causing the (bit) surprising result.

Now we will apply a trick, we insert an invisible character after each mark, in this way the shuffle effect will be prevented. Uncomment the line with the insertion of "\u200b", and look at the result, the content now is "12".

We will examine step by step what happens:

(We use "~" for the invisible char.)

We can see that this trick helps, the shuffle effect does not occur. This has an significant advantage, we have an exact editor control, because marks now can be used like parentheses, and this is not possible without the new feature (or without this trick with the invisible char).

Now we examine the behavior of the new option, what happens if we consider marks simultaneously as invisible characters:

As we can see it works as expected, it works in the same way as with the invsible char trick (which prevents the shuffling).

The shuffle effect isn't noticeable with a normal editor flow, filling the content from left to right, line by line. But programmed editor control is a crux without the new feature. And the example with the invisible char trick demonstrates that the steadymarks feature is the natural way how the marks should behave.

This feature is used in chess application Scidb. This application has a sophisticated editor control for the display of chess games, with multi-level folding, and some more specials (the Tk widget is very powerful). It began with this invisible char trick.

Also the chess application Scid is using (nearly) all the features of the editor, but here the shuffle effect is breaking the layout (this occurs when appending moves, and then doing some operations like deletions, changing a variation to mainline, appending more moves, and so on).

The old behavior with marks is a bit archaic, so I would replace it with the new concept (as default), but it may happen - although this is in general not expected - that an existing application breaks, therefore I did not replace the old concept.

The effort for the implementation of this feature was minimal, but the benefit is huge.

With this new option we need an extension in description of mark set.

This command has also been extended with an optional third parameter, the direction of the gravity. It is a very common case that the gravity will be specified after setting the mark, now this can be done with one command (this also results in a slight performance improvement when option -steadymarks is not enabled).